Paleobiology is the study of the origin, structure, physiology, behavior, ecology, and evolution of extinct organisms, and fossil eggs and eggshell can reveal useful information about the paleobiology of
the animals that laid the eggs. Nest structures,
whole eggs with embryos,
associated skeletons, and the encasing sediments can all aid in answering paleobiological
questions. The preservation of nests and associated bones (especially embryos)
are far less common in the fossil record than eggshell fragments, but each discovery
of eggs with associated material sheds more and more light on the biology of
extinct animals.

Much of the scientific literature about fossil eggshell tends to focus on dinosaurs.
This is largely due to the popular appeal of dinosaurs, as well as the preservation
potential of dinosaur eggs. Dinosaur nests and nesting grounds have been discovered
in floodplain sediments,
which commonly preserve fossils from terrestrial environments.
Spectacular dinosaur nesting localities have been found in Montana, Argentina,
China, India, France, Spain, and Mongolia, advancing the knowledge of the reproduction
and nesting habits of several dinosaur species.

Paleobiological information from eggshell structure
The surface of an egg is not perfectly smooth. Often fossil eggshell is textured
with ridges or nodes, and all eggshell is perforated by a network of pores which
allow gases (oxygen, water vapor, and carbon dioxide) to be exchanged between
the embryo and the environment outside the egg. These features can provide information
about both nesting behavior and climate. For example, some researchers speculate
that a surface texture of nodes and ridges can help in preventing plant debris
(often used to cover the eggs) from plugging the pores, thereby protecting embryos
from suffocation. Porosity and water vapor conductance rate (related to the exchange
of gases between the embryo and environment and thus eggshell porosity) have
also been related to amniote nesting behavior and may indicate whether eggs were
buried in sediment or left exposed. Lower porosity and water vapor conductance
have been associated with a more bird-like open (uncovered)
incubation of
eggs, while high porosity and conductance rates have been associated with the
non-avian reptile-like behavior of burying egg clutches. Low porosity has also been attributed
to arid climates, as minimal water loss from an egg would be imperative for embryo
survival.

Paleobiological information from nests (Fig. 1)

Click any image on this page to see an enlargement.

Figure 1. Cast of a Troodon nest from the Late Cretaceous of Montana. Note the circular arrangement of eggs. Specimen UCM 239, photograph CO96.18.

Scientists have made inferences about the reproductive physiology
and egg-laying strategies of dinosaurs by looking at the arrangement and orientation
of eggs in a nest. The arrangement of eggs has been used to infer the number
of functioning ovaries in the egg-layer. For example, eggs that appear to be
paired in a nest suggest that the egg-layer had two functioning ovaries, in
which two eggs were formed at the same time and then laid one after the other  perhaps
followed by a period of inactivity before the next pair was laid. Unlike non-avian
reptiles that possess two ovaries and oviducts and lay eggs all at one time,
extant birds have only one functioning ovary (the other one was apparently
lost as an adaptation to reduce body weight for flight) and lay one egg at
24 hour or greater intervals. Both ovaries and oviducts may have functioned
simultaneously while forming only one egg at a time in their extinct theropod relatives, a condition not present in any extant amniotes. This shows how data regarding dinosaur reproductive physiology
can shed light on the relatedness of certain dinosaur groups to birds or crocodiles
(the two closest extant relatives of the non-avian dinosaurs).

Patterns of eggshell preservation in a nest have also been linked to nesting
behavior and parental care. Lots of broken eggshell in a nest has been interpreted
as evidence that hatchling dinosaurs remained in the nest for extended periods;
the eggshell may be broken because it was trampled by nest-bound hatchlings.
That the hatchlings spent significant time in the nest after hatching implies
that adults of at least some dinosaur species provided a degree of parental
care for their young.

Additionally, adult specimens have been found in nest structures along with hatchlings
and juveniles of the same species. The preservation of adults and juveniles
of the same species together provides strong evidence of parenting behavior.
Spectacular examples of this have been found in Montana and Mongolia (among
other places; see "Additional readings").

Paleobiological information from embryos (Fig. 2)
Eggs that are found with their associated embryos provide the only opportunity
to absolutely determine which eggshell type was characteristic of a specific clade of
extinct animal. Sometimes the embryonic material may be identifiable to a particular
species (especially if juvenile and adult skeletons are also in the area), and
sometimes nothing more may be known than a broad taxonomic group  like "Sauropoda" or "Pterosauria." Unfortunately, embryonic remains are very
rare in the fossil record  the bones are small, fragile, usually incompletely ossified,
and thus easily destroyed. Eggs and embryonic bones are only preserved under
exceptional depositional conditions; fossil embryos have been reported from dinosaurs,
turtles, birds, and pterosaurs.

The study of embryonic and juvenile remains has proven useful in understanding the ontogeny (development
and growth) of extinct animals. Characteristics such as the size of the embryo
relative to adult body size and the degree to which the embryo's bones are
developed can be compared to embryos of extant animals. This technique has
been used when comparing non-avian dinosaurs and birds. The degree of ossification
(bone formation) at hatching has also been used to interpret dinosaur parental
behavior  if a hatchling had poorly developed bones that prevented it
from immediately leaving the nest, the adults would have had to provide for
the newly hatched young.

Paleobiological information from pathological eggs (Fig. 3)

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Figure 3.A. Thin section of a multi-layered pathological dinosaur
eggshell from the Late Cretaceous of France. The original deposition of calcite
eggshell was interrupted and then resumed, producing two distinct layers. Specimen
UCM 571-1, photomicrograph M398.18. B. SEM photomicrograph of a multi-layered
pathological dinosaur eggshell from the Late Jurassic of Utah. Specimen UCM 488-3,
photomicrograph G77.2.

Pathological eggs
are those with a shape, size, or eggshell structure different from normal eggs. One of the more common pathologies is multi-layered eggshell.
Eggs with multiple shell layers have been documented in modern and fossil eggs,
most commonly in turtles and dinosaurs. In these cases, a second eggshell layer
is deposited on top of the original shell; these shell layers may be separated
by membrane or
yolk (though this is not preserved in the fossil record). Pathological shell
deposition in extant egg-layers is attributed to physiological and/or environmental
stress experienced by the female  such as drought, flooding, disease,
or high population density. Retention of the egg under such stresses leads to
deposition of one or more additional eggshell layers.

It is not easy to deduce the reproductive habits of extinct animals that produced
pathological shell. Pathological eggs preserved in situ (undisturbed
and in original position) in a nest are rare, and have never been found with
skeletal remains. It is inferred that a multilayered eggshell
is fatal to embryos because the pores do not align between the shell units
and embryos would asphyxiate. Before multilayered eggs were found in extant
avian eggshell, pathological eggs were interpreted to represent reptile-like
reproduction. However, this is no longer accepted. Pathological eggs are most
common in turtles, but they likely do not provide an appropriate analog to
dinosaur reproduction due to differences in evolutionary lineages.

Paleobiological information from preservation
Although useful paleobiological information can be gleaned from eggs and eggshell,
the preservation of eggshell is relatively uncommon in most deposits, and the
preservation of nests and embryos is even rarer. Nevertheless, information about
the paleobiology and paleoecology of extinct animals can be gained by studying
the conditions under which these fossils were preserved. Whether eggshell has
been transported to the site of preservation or is in situ plays an important
role in the amount of information available from a deposit. Accumulations of
eggshell fragments near a nesting area might have been derived from multiple
eggs originally laid in the nests. In situ remains reveal the preferred nesting environments of extinct animals. Alternatively, accumulations of eggshells
may have been washed in by rivers or during flooding events, may have belonged
to other organisms that shared the nesting area, or may even have been the remains
of food eaten by recently hatched nestlings. Although transported eggshell cannot
provide as much paleobiological information as in situ eggshell, it offers
information on the ancient river systems, such as stream power and geochemical
conditions.

Cases of exceptional preservation of fossil eggs are particularly informative. Organic matter usually decays during fossilization, but preservation of organic matter within eggshell increases our understanding of the microstructure (internal morphology) and taphonomy of fossil eggshell. Preservation of tissues inside eggs is also informative. One dinosaur nesting site in Argentina has produced fossil egg remains with embryonic skin impressions as well as shell membrane. In other rare cases, eggs have been found preserved in situ within the body cavities of dinosaurs and turtles, providing unique opportunities to study reproductive physiology.

Paleobiological information from depositional environment
The nature of the depositional environments where eggs are found can also reveal
information about the breeding behavior of nesting animals. Discoveries of multiple
layers of nests within the same area have led researchers to suspect that some
localities have been used by the same species of animal over multiple breeding
seasons. This not only indicates gregarious behavior (animals tending to form
groups) by some species, but also suggests "site fidelity," returning
to the same nesting sites over multiple years. However, it should be noted
that these terms can have a different meaning when applied to the fossil record
than when applied to the ecology of modern taxa. In the latter, the same
animals typically return to a nesting site within a short time interval (nesting
season or years). In the fossil record, egg-bearing horizons may be separated
by several meters of sediment that might represent hundreds to thousands of years.

Additionally, the nature of the sediments in which eggs and eggshells are
deposited can reflect environmental conditions in which extinct animals lived.
The presence of caliches indicates
wet and dry seasons, and other evaporite minerals
may indicate that eggs were deposited in arid or semi-arid environments. Paleosols (ancient
soils) containing fossil eggshell can provide paleoenvironmental information
such as temperature, precipitation, hydrological conditions, etc. Other fossils
associated with eggshell may reveal other animals that shared ancient environments
with the egg-layers.